1. Field of the Invention
The present invention relates to the integration of micro-optical elements on a semiconductor substrate, particularly glass-material optical components such as lenses, beam splitters, mirrors and polarising elements that are to be mounted on a silicon substrate.
2. Discussion of the Background Art
Opto-electronic devices, for example laser transmitter and receiver units for use in fibre optic transmission systems, and laser diode units for use in applications as diverse as optical data storage and laser range finding systems, often combine a solid state optical source or detector with a number of glass-material micro-optical elements, such as a collimating lens, beam splitter, polarising element or optical waveguide. These elements may have to be secured in a particular orientation with respect to an opto-electronic element. In particular, the position of a collimating micro-lens or a focussing micro-lens may need to be held to an accuracy of less than 1 xcexcm for up to 25 years for the performance of a product to be maintained.
Because silicon processing is a well-established and economical technology, micro-optical elements are often secured to a machined and etched silicon substrate, formed as one of many from a larger silicon wafer. Electrical connections and insulating areas can also be conveniently formed on such a substrate by well-known means. One way in which a bond can be made between the micro-optical elements and the substrate is by using an epoxy adhesive, for example an ultraviolet or thermal curing epoxy adhesive. Although it is possible to obtain good long term dimensional stability with such adhesives, a problem has been noted that such cured epoxy adhesives can move or deform in subsequent processing of the substrate, or due to temperature excursion experienced during normal operation of the product, and environmental factors. For example, when a solder connection is made to a contact pad on the substrate, the cured adhesive can be raised to a temperature of 320xc2x0 C. to 350xc2x0 C. This can cause the position of the secured micro-optical element to shift, thus affecting adversely the performance of the opto-electronic device.
In many applications it is also necessary that the opto-electronic element is able to survive mechanical shocks that could dislodge a micro-optical element secured to the substrate. In practice, the micro-optical element should be able to withstand a static force of at least about 0.5 N.
It is an object of the present invention to provide a micro-optical device, and a method for forming such a device, that addresses these issues.
According to the invention there is provided a micro-optical device, comprising a semiconductor material substrate, and at least one glass-material micro-optical element mounted on said substrate, characterised in that the device includes a glass-material stand interposed between said micro-optical element and said substrate, said stand being fuse-bonded to both the substrate and the micro-optical element.
Also according to the invention, there is provided an opto-electronic device, comprising at least one opto-electronic element and a micro-optical device, characterised in that the micro-optical device is according to the invention and in that the opto-electronic element is secured to said substrate and coupled optically with said at least one micro-optical element.
It has been found that the fuse-bonding of the stand to the adjacent substrate and micro-optical element forms a secure and stable mount of the micro-optical element to the substrate, with very low creep over time. The glass transition or melting point of the glass material stand will be related to a suitable fuse-bond temperature and bonding time. The fuse-bond temperature can be chosen to be lower than that of the micro-optical element, so that the element is essentially unaffected by the fuse-bonding process. In addition, the fuse temperature can be chosen to be sufficiently higher than temperatures expected in subsequent processing, for example from soldering of electrical contacts, so that the fuse-bonded stand is essentially unaffected by such subsequent temperatures.
In a preferred embodiment of the invention, the micro-optical element is a lens arranged to couple optical radiation into (or out of) one or more opto-electronic elements.
Prior to formation of the micro-optical device, the stand may be a partially fused glass-material frit. This is a convenient way to form an glass-material stand that has a particular desired shape. The frit is sufficiently robust so that it may be handled, for example by a micro-manipulator such as a vacuum pick-up tool, and then placed in position on the substrate. The fuse-bonding process may then further fuse together the frit, so that this becomes a fused frit.
In a preferred embodiment of the invention, the substrate has a recess in a surface thereof, said recess being bounded by one or more walls. The stand may then conveniently be dimensioned to fit within the recess and be positioned by one or more walls of said recess.
In its simplest form, the recess may be a circular recess, in which case there may be just one circular wall. In a preferred embodiment of the invention the recess is in a groove with opposite side walls that position the stand in a first direction. The recess and the stand may then have therebetween an alignment feature that aligns the stand in a second direction transverse to the first direction.
For example, the alignment feature may be a ridge on one of the stand or the recess, and a groove matching the ridge on the other of the stand or the recess.
The invention also provides a method of forming a micro-optical device, said device comprising a semiconductor material substrate, at least one glass-material micro-optical element and at least one glass-material stand, characterised in the method comprises the steps of:
i) bringing together the micro-optical element and the substrate so that a stand is interposed between the micro-optical element and the substrate;
ii) heating the stand so that this becomes fuse-bonded to both the substrate and the micro-optical element, thereby mounting said micro-optical element to the substrate; and
iii) then allowing the stand to cool down so as to fix the bond between the stand and said micro-optical element and between said stand and the substrate.
Step ii) may then be accomplished by applying heat through the substrate towards the stand. For example, a heater may be affixed to a rear surface of the substrate, so that heat conducted through the substrate causes the fuse-bonding of the stand.
The invention further provides a method of forming an opto-electronic device, said opto-electronic device comprising at least one opto-electronic element and a micro-optical device, characterised in that said micro-optical device is formed according to the method of forming a micro-optical device according to the invention, and the opto-electronic device is then formed by the step of securing said opto-electronic element to said substrate so that said opto-electronic element is coupled optically with at least one of said fuse-bonded micro-optical elements.